This invention relates to a process and apparatus for producing readily polymerizable vinylaromatic compounds. More particularly, this invention relates to a process and apparatus for inhibiting the formation of undesired accumulations of polymeric material in a distillation apparatus during distillative purification of vinylaromatic monomers.
Vinylaromatic monomers, such as styrene, alpha-alkylstyrene, vinyltoluene, divinylbenzene and the like, are important for their ability to form useful polymeric materials. These compounds are typically prepared by catalytic dehydrogenation of alkylaromatic compounds having corresponding carbon chains. The crude product of the dehydrogenation reaction, however, is a mixture of materials comprising in addition to the desired vinylaromatic monomer, various alkylaromatic compounds as well as oligomers of the desired monomer. These other substances must be separated from the vinylaromatic monomer to obtain a commercially acceptable product.
The usual method for separating a desired vinylaromatic monomer from the dehydrogenation product mixture is to pass the mixture through a distillation train in which lower boiling materials are first separated and then the desired monomer is distilled from the higher boiling materials. Such distillative separations are complicated by the fact that the tendency of the monomer to polymerize increases with increasing temperature. Consequently, as the mixture is heated to distill it, the formation of undesired polymer increases and the yield of desired monomer decreases.
Masses of unwanted polymeric material thus accumulate in the distillation apparatus. The continuing accumulation of these undesired polymeric materials requires that a distillation apparatus used to purify vinylaromatic monomers be periodically shut down and cleaned of the fouling polymer. As the polymer is typically a dense hard material, considerable difficulty is encountered in cleaning the distillation apparatus. The need for periodic cleaning contributes substantially to operating costs, and capital costs are also increased because additional distillation capacity must be constructed in order to compensate for the down time of the distillation apparatus.
Not only is the yield of monomer decreased and the productivity of equipment lessened by reducing effective capacity and by requiring periodic shutdowns for cleaning, but the insoluble residues produced by the undesired polymerization of the monomer constitute a waste material which must be disposed of. Governmental restrictions on waste disposal make this an ever increasing problem.
Various measures have been utilized to minimize the undesired polymer formation and the resulting fouling of the distillation column. Vacuum distillation has been used to reduce the temperature to which the feed mixture must be heated. While this is helpful in reducing the formation of undesired polymeric material, substantial amounts of polymer still are formed.
Polymerization inhibitors have been added to the feed mixture. Known inhibitors further reduce the formation of undesired polymer, but still are not totally effective. Moreover, such inhibitors may be expensive and contribute substantially to the production costs for the vinylaromatic monomer.
A particular problem arises in the distillation apparatus in areas of high concentrations of liquid condensate and low vapor velocity, such as adjacent the tray bottoms and along the downcomers. Vapor phase active inhibitors are not totally effective in suppressing the formation of deposits in such areas because the inhibitor is restricted in mixing with the condensing vapor due to low vapor velocity. Because liquid phase active inhibitors are not carried with the monomer as it vaporizes, the liquid condensate is largely free of these polymerization inhibitors. Condensate droplets which form against the cooler tray bottoms, thus, may polymerize and solidify before they grow large enough to drop into the underlying tray. Polymerization also tends to occur in the downcomer since there is a high concentration of liquid monomer condensate, little or no polymerization inhibitor and a low vapor velocity. Consequently, monomer droplets can attach to the downcomer walls, and as the condensate droplets remain on the wall, polymerization occurs and the column becomes fouled.
U.S. Pat. No. 4,207,276 discloses placing a polytetrafluoroethylene coating on the inner surfaces of bubble-caps and the inner and outer surfaces of the bubble cap necks to prevent clogging. The principal function of the coating is to facilitate ready separation of fouling deposits after they have formed. Preventing formation of the deposits in the first place is not a consideration. These are high vapor velocity areas. A high vapor velocity not only allows for mixing of inhibitor with the vapors, but also discourages attachment of condensing vapor to the inner surfaces of the column due to vapor agitation, and further acts as a sweep to remove attaching condensate droplets from the column surfaces. Because of this above-mentioned vapor action, however, the inhibition of polymerization in low vapor velocity areas presents a different problem, and different considerations come into play.
Despite the efforts of the prior art, there remains a substantial need for improved methods and apparatus for inhibiting the initial formation of undesired polymeric residues in distillation apparatus used to purify vinylaromatic monomers, particularly in areas such as on the tray bottoms and in the downcomers, where high concentrations of condensate and low vapor velocities are encountered.